宍道湖西部域の湖底地形と堆積層

The western part of Lake Shinji was surveyed by SH-20 echosounder to determine the topography and sediment layers to a depth of about 20m．Here，the Hii River Delta develops eastward，forming a subaqurous channel with natural levees on both sides．By comparing the present results with the bottom topography surveyed during 1962-63 by the Geographical Survey Institute，the Hii River Delta is shown to have prograded eastward about 1.5km．Several reflected layers are discovered in the bottom sediments．These layers increase in number toward the river mouths．To determine the composition of these reflectors，a 1.4m long sample was taken by gravity coring and was studied by soft-X Ray and grain size analyses．Two coarse layers were identified，which correspond to the reflected layers．These two layers are assinged to be flood deposits at A. D. 1934 and A. D. 1635，respectively．The rate of sedimentation is evaluated at 2-3mm／y（before 1635），3-4mm／y（between 1872 and 1934）and 7mm／y（after 1934）．